in Javascript, I am creating an object like this:
var testObject = {
value: "this is my initial value",
setup: function() {
value: "foo"
}
};
Now, I would like to be able to instantiate this object, so I am trying this:
var myFirstObject = new testObject();
var mySecondObject = new testObject();
so that when I call .setup() it will change the value only for that referenced newly created object. How can I achieve this? This code does not seem to work.
You don't instantiate objects, you instantiate functions.
var testObject = function() {
this.value = "this is my initial value";
this.setup = function() {
this.value = "foo";
}
}
var myFirstObject = new testObject();
var mySecondObject = new testObject();
EDIT:
As per your comment, here's how you would bind to the DOM using functions on this object:
document.getElementById('idOfElem').addEventListener(
'click', myFirstObject.clickHandler);
Bear in mind that you won't have any guarantee that the click handler will be executed in the context of your object (i.e. in your click handler, this might not be your testObject instance). If your clickHandler intends to modify the object's instance variable in any way, it's better to ensure the context like so:
document.getElementById('el').addEventListener('click',
function() {
myObj.handleClick.apply(myObj, arguments);
});
You have numerous problems with your code. Firstly, you are trying to instantiate something, by calling a constructor function. Your testObject is not a function, so you'll cause a type error. You need to change testObject to be something along these lines:
var TestObject = function () {
this.value = "this is my initial value";
};
TestObject.prototype.setup = function () {
this.value = "foo";
};
Notice that I've used an uppercase T in that identifier... that's just best practice for a constructor function. Also notice how I've defined the setup method on the prototype. This is much more efficient than defining it as a property of the instance (with this.setup) since only one copy of the function needs to exist in memory.
Now that TestObject is a function it can be instantiated by calling it with the new operator:
var myFirstObject = new TestObject();
var mySecondObject = new TestObject();
When you call the setup method on an instance of TestObject, it will apply to that instance. In other words, the value of this inside the setup method will refer to the instance on which the method has been called:
myFirstObject.setup();
console.log(myFirstObject.value); // 'foo'
console.log(mySecondObject.value); // 'this is my initial value'
You have incorrectly defined your constructor. Try this:
function testObject() {
this.value = "this is my initial value";
this.setup = function() {
this.value = "foo"
}
};
You can then call new testObject().
The object notation your using is something you can compare with a static class.
Here is the code for what you're trying to achieve:
var testObject = function(val) {
this.value = "This is my initial value",
if (arguments[0]) {
this.value = val;
}
};
var first = new testObject(); //uses initial value
var second = new testObject("hi"); //value = hi
If you'd like to write classes using this notation take a look at this: http://ejohn.org/blog/simple-javascript-inheritance/
function yourObject(value, setup) {
return {
value: value,
setup: setup
};
}
var myFirstObject = new yourObject('a', function(){});
var mySecond = new yourObject('b', function(){});
Related
I'm always getting Cannot set property 'saySomething' of undefined but why?
Am I making a mistake somewhere?
var Person = new Object();
Person.prototype.saySomething = function ()
{
console.log("hello");
};
Person.saySomething();
Debugging tip: You get this ..of undefined errors when you try to access some property of undefined.
When you do new Object(), it creates a new empty object which doesn't have a prototype property.
I am not sure what exactly are we trying to achieve here but you can access prototype of function and use it.
var Person = function() {};
Person.prototype.saySomething = function() {
console.log("hello");
};
var aperson = new Person();
aperson.saySomething();
The prototype property exists on functions, not on instantiated objects.
var Person = new Object();
console.log(Person.prototype); // undefined
var Person2 = function () {}
console.log(Person2.prototype); // {}
This is useful because things put on the prototype of a function will be shared by all object instances created with that function (by using new).
var Person = function() {};
Person.prototype.saySomething = function() {
console.log("hello");
};
console.log(
new Person().saySomething === Person.prototype.saySomething // true. they are the same function
);
If all you want is to add a method to the person object, there's no need for a prototype:
var Person = {};
Person.saySomething = function() {
console.log("hello");
};
Person.saySomething();
You can even use object literal syntax:
var Person = {
saySomething: function() {
console.log("hello");
}
};
Person.saySomething();
i was trying out some code thought of posting it, might help others.
<script>
var MODULE = {};
MODULE = (function (my) {
my.anotherMethod = function () {
console.log("hello ");
};
my.newMethod = function(){
console.log("hi new method ");
}
return my;
}(MODULE));
MODULE.anotherMethod();
MODULE.newMethod();
</script>
And please not var MODULE ={}, if this is not initialized with {} then it give cannot set property.
I know i am late to the party but as you see there is no satisfying answer available to the question so i am providing my own.
In your case when you write
var Person = new Object();
you are creating an instance of Object type.
You can add a property using prototype property to the Object, not to the instance of Object.which you can use by the instance laterly.
so you can define like
Object.prototype.saySomething = function ()
{
console.log("hello");
};
now you can call it like this.
Person.saySomething();
You can check here.
var Person = function(name) {
this.canTalk = true;
this.name = name;
};
Person.prototype.greet = function() {
if (this.canTalk) {
console.log('Hi, I am ' + this.name);
}
};
bob = new Person('bob');
bob.greet();
I have been trying to learn the ins and outs of Javascript's inheritance structure and I came across this problem. I'm trying to create several subclass objects and assign values to them immediately by passing a variable upon their creation.
For instance, below the parent class GamePiece receives a random property as a variable upon its creation and sets it as myProperty which works fine of course when creating a new GamePiece object. However, if I wanted to set that variable on the creation of a Pawn object, it does not pass into its parent and remains unset. The obvious fix is to just define the variable again on the subclass but that, correct me if I'm wrong, seems to defeat the purpose of defining a parent class. You can also successfully set the variable by passing the parameter as Pawn.prototype = new GameObject("foo"); but that doesn't help when creating multiple Pawn() objects. Is there a common way of doing this that I am missing?
var GamePiece = function (randomProperty) {
this.myProperty = randomProperty || "never set";
this.print = function () {
console.log(this.myProperty);
}
}
var Pawn = function (randomProperty) {
this.print = function () {
console.log(this.myProperty);
}
}
//Setting a value on creation
piece = new GamePiece("foo");
piece.print(); // Produces "foo" naturally
//Setting the prototype
Pawn.prototype = new GamePiece();
//Try to pass value through the creation of subclass
pawn = new Pawn("foo");
pawn.print(); // Produces "never set"
You must call you parent class in context of current this, using call or apply method:
var GamePiece = function (randomProperty) {
this.myProperty = randomProperty || "never set";
this.print = function () {
console.log(this.myProperty);
}
}
var Pawn = function (randomProperty) {
Game.call(this, randomProperty);
// or Game.apply(this, [randomProperty]);
}
But much it's better to keep methods in prototype. So next code will be better:
var GamePiece = function (randomProperty) {
this.myProperty = randomProperty || "never set";
//... some another properties initialization
};
GamePiece.prototype.print = function () {
console.log(this.myProperty);
};
var Pawn = function (randomProperty) {
Game.call(this, randomProperty);
// or Game.apply(this, [randomProperty]);
//... some Pawn properties initialization
};
Pawn.prototype = Object.create(Game.prototype, { constructor: { value: Pawn }});
Pawn.prototype.someMethod = function() {
// Some Pawn method logic
};
But ES6 is coming (will become recommendation in June 2015), so you can start preparation for using them. See here, here and here
I have this code
var testObject = new TestObject();
testObject.init();
function TestObject() {
this.init = function() {
var uiCtrlDescriptors = {
"htmlElem": "elem",
"controller" : new Controller(this.htmlElem)
}
}
}
and I don't understand why this is equal to testObject inside this line:
"controller" : new Controller(this.htmlElem)
instead of being equal to uiCtrlDescriptors object?
That's just how this works. Whenever you do:
obj.some_function();
then some_function runs with this set to obj.
I like that in javascript, I can create a function, and then add further methods and attributes to that function
myInstance = function() {return 5}
myInstance.attr = 10
I would like to create a class to generate these objects. I assume I have to inherit from the Function base class.
In other words, I would like to:
var myInstance = new myFunctionClass()
var x = myInstance()
// x == 5
But I don't know how to create the myFunctionClass. I have tried the following, but it does not work:
var myFunctionClass = function() {Function.call(this, "return 5")}
myFunctionClass.prototype = new Function()
myInstance = new myFunctionClass()
myInstance()
// I would hope this would return 5, but instead I get
// TypeError: Property 'myInstance' of object #<Object> is not a function
I also tried the more complicated (and more proper?) inheritance method found here: How to "properly" create a custom object in JavaScript?, with no more luck. I have also tried using the util.inherits(myFunctionClass, Function) found in node.js. Still no luck
I have exhausted Google, and therefore feel that I must be missing something fundamental or obvious. Help would be greatly appreciated.
Your trying to inherit from Function. This is a right pain to do. I suggest you do the following instead
Live Example
var Proto = Object.create(Function.prototype);
Object.extend(Proto, {
constructor: function (d) {
console.log("construct, argument : ", d);
this.d = d;
// this is your constructor logic
},
call: function () {
console.log("call", this.d);
// this get's called when you invoke the "function" that is the instance
return "from call";
},
method: function () {
console.log("method");
// some method
return "return from method";
},
// some attr
attr: 42
});
You want to create a prototype object that forms the basis of your "class". It has your generic methods/attributes. It also has a constructor that gets invoked on object construction and a call method that gets invoked when you call the function
var functionFactory = function (proto) {
return function () {
var f = function () {
return f.call.apply(f, arguments);
};
Object.keys(proto).forEach(function (key) {
f[key] = proto[key];
});
f.constructor.apply(f, arguments);
return f;
}
}
A function factory takes a prototype object and returns a factory for it. The returned function when called will give you a new function object that "inherits" from your prototype object.
var protoFactory = functionFactory(proto);
var instance = protoFactory();
Here you create your factory and then create your instance.
However this isn't proper prototypical OO. we are just shallow copying properties of a prototype into a new object. So changes to the prototype will not reflect back to the original object.
If you want real prototypical OO then you need to use a hack.
var f = function () {
// your logic here
};
f.__proto__ = Proto;
Notice how we use the non-standard deprecated .__proto__ and we are mutating the value of [[Prototype]] at run-time which is considered evil.
JS does not allow a constructor to return a function, even though functions are objects. So you cant have an instantiation of a prototype that is itself executable. (Am I right in this? please correct if I'm not, it's an interesting question).
Though you could do a factory function:
var makeCoolFunc = function() {
var f = function() { return 5 };
f.a = 123;
f.b = 'hell yes!'
return f;
};
var func = makeCoolFunc();
var x = func();
You can extend Function and pass the wanted function body as String to the super constructor. The context of the function can be accessed with arguments.callee.
Example for an observable Attribute class:
export default class Attribute extends Function {
constructor(defaultValue){
super("value", "return arguments.callee.apply(arguments);");
this.value = defaultValue;
this.defaultValue = defaultValue;
this.changeListeners = [];
}
apply([value]){
if(value!==undefined){
if(value!==this.value){
var oldValue = this.value;
this.value=value;
this.changeListeners.every((changeListener)=>changeListener(oldValue, value));
}
}
return this.value;
}
clear(){
this.value=undefined;
}
reset(){
this.value=this.defaultValue;
}
addChangeListener(listener){
this.changeListeners.push(listener);
}
removeChangeListener(listener){
this.changeListeners.remove(listener);
}
clearChangeListeners(){
this.changeListeners = [];
}
}
Example usage:
import Attribute from './attribute.js';
var name= new Attribute();
name('foo'); //set value of name to 'foo'
name.addChangeListener((oldValue, newValue)=>{
alert('value changed from ' +oldValue+ ' to ' +newValue);
});
alert(name()); //show value of name: 'foo'
name('baa'); //set value of name to new value 'baa' and trigger change listener
Quick and strange question:
I have an object (in this example is small but in the project is larger):
var myObject = {
hello: 1, // easier I think
'hey.ya': 5 // quite impossible but the first option is valid too
}
then I want to pass somehow to a function and use "hello" for example in a closure like this
function x(){
// my closure
return function(){this.init = function(){alert(hello)}, this.heyYa = function(){alert(/* I do not know how to call the other hey.ya variable */)}}
}
var myClass = x(), instance = new myClass(); instance.init();
thanks!
You need to use the myObject
var myObject = {
hello: 1,
'hey.ya': 5
}
function x(obj){
return function(){
this.init = function(){
alert(obj.hello)
},
this.heyYa = function(){
alert(obj['hey.ya'])
}
}
}
var myClass = x(myObject);
var instance = new myClass();
instance.init(); // alerts '1'
instance.heyYa(); // alerts '5'